1. Field of the Invention
The present invention relates to a lens driver circuit and, more particularly, to a lens driver circuit with ringing compensation.
2. Description of the Related Art
A voice coil motor (VCM) is a well-known structure that includes a body, a coil that is attached to the body, and a permanent magnet that surrounds the coil. A VCM is operated by passing a current through the coil. The current passing through the coil generates a magnetic field which reacts with the magnetic field from the permanent magnetic to move the coil and the body.
VCMs were originally developed as a means to drive a loudspeaker cone, which functions as the body of the VCM. When a current that represents an audio waveform is passed through the coil, the resulting current-generated magnetic field reacts with the magnetic field from the permanent magnet to move the loudspeaker cone which, in turn, generates sound pressure waves that reproduce the original sound.
One well-known variation of the original VCM is to include a spring that is attached to the body. When a spring is utilized, a current can be passed through the coil so that the reacting magnetic fields move the coil and the body to compress the spring, with the position of the coil and the body determined by balancing the magnetic and spring forces. The energy stored in the compressed spring can then be used to return the coil and the body to a previous position, thereby saving energy.
Spring-based VCMs are frequently used in low-power electronic applications. For example, in a compact camera application, such as the compact camera modules used in smartphones, the magnitude of the current passing through the coil can be incrementally increased. The incremental increase in current causes the coil and the body to incrementally move and step a lens, which is attached to the body of the VCM, through a number of focal positions to find an in-focus position for the lens. The movement of the combined structure, which includes the coil, the lens, and the body, compresses the spring, which is then used to return the combined coil, lens, and body structure to a previous position.
One problem with using a spring-based camera VCM to move a lens through a number of focal positions is that when the coil, lens, and body structure reaches a desired position, the momentum of the moving structure causes the spring, and thereby the coil, lens, and body structure connected to the spring, to oscillate around the desired position for a period of time before stopping at the desired position. This oscillation around the desired position is commonly known as ringing.
The time required for the oscillation or ringing to stop can significantly increase the time required to find an in-focus position for the lens when the lens must step through a number of positions in a very short period of time. Thus, there is a need for an approach that substantially reduces the oscillation or ringing.